This invention relates to fuel injection nozzles used in diesel engines, and particularly to injection nozzles that are used in mechanical injectors of the type known as EMD injectors, originally manufactured by Diesel Equipment Division of General Motors for Electro Motive Division of General Motors. As used herein, xe2x80x9cEMD-type injectorsxe2x80x9d refer to mechanically operated devices, as distinguished from solenoid-operated devices (also made by the same manufacturer).
EMD-Type Injectors.
EMD-type injectors include a nozzle body which houses a nozzle valve and terminates in a nozzle tip. The seat for the nozzle valve is formed at or near the nozzle tip. When the valve is open (when its distal end is raised from the valve seat) incoming pressurized fuel flows to a small feed chamber or xe2x80x9csac,xe2x80x9d located just below the seat and within the tip, and is distributed by the sac to spray holes formed in the wall of the nozzle tip. The spray holes lead into the engine chamber where the fuel is atomized.
The nozzle valve is biased to closed position by a valve spring. This spring is of the coil-spring type and is contained within a spring cage having a spring chamber of generally cylindrical shape. The spring cage is stacked just above (upstream of) the nozzle body. The diameter of the spring chamber (the inside diameter of the spring cage) is only slightly larger than the outside diameter of the spring, such that the spring fits snugly within the spring chamber, but with sufficient clearance to allow the spring to freely compress and expand therein as the nozzle valve opens and closes. The spring force is transmitted axially through the stem portion of the nozzle valve to bias the nozzle valve to seated, closed position until the bias of the spring is overcome by pressure of incoming fuel acting on a conical differential area of the nozzle valve. This latter action forces the nozzle valve in the opening direction against the bias of the spring.
A disc type check valve for preventing reverse flow of the fuel is contained in a check valve cage stacked just above (upstream of) the spring cage. Additional elements are stacked still further upstream, including the bushing of a plunger-and-bushing assembly for pressurizing the diesel fuel during each injection cycle.
The nozzle body, spring cage, check valve cage and other elements are stacked one above the other within a housing nut. The housing nut is itself threadedly connected on a boss on an assembly block, and when this threaded connection is tightened down, the stacked elements are firmly secured in their stacked relationship.
Spring Seats in EMD-Type Injectors.
A particular characteristic of an EMD-type injector is the design of the spring seat. This element couples the spring to an extension of the nozzle valve, thereby accomplishing the transmission of compressive forces between the spring and the nozzle valve. The spring seat has a cylindrical spring seat stem which is surrounded by and relatively snugly received within the lower end of the coil spring, but again with sufficient clearance to allow the spring to freely compress and expand along the stem as the nozzle valve opens and closes. The spring seat also has an annular head that is coaxial with the spring seat stem. The head is foreshortened, being axially shorter than it is wide, so that the overall shape of the spring seat is similar to a mushroom with its stem and head, but inverted so the head is below-the stem, i.e., with respect to the position and orientation of the spring seat in the overall nozzle valve assembly, the foreshortened head forms the distal end of the spring seat and the stem forms the proximal end.
The spring seat has an annular flat face formed on the proximal side of its head against which the lower or distal end of the coil spring bears. This face also may be referred to as the spring-receiving face. The end of the coil spring is ground to provide area contact between the spring and the flat face around a substantial annular extent of the flat face, and preferably around a majority of said annular extent. The spring-receiving or flat face is perpendicular to the sidewall of the spring seat stem and meets it at a first annular juncture. The coil spring is unrestricted against creeping in a rotating motion around its central axis as it compresses and expands.
A central head recess extends axially within the annular head and coaxially therewith to a depth which is a considerable portion of the total thickness of the head at is thickest point (the total thickness being the axial distance from the distal end to the plane of the annular flat face). This recess has an annular sidewall and terminates in a circular end wall perpendicular to the sidewall and meeting the sidewall at what may be referred to as a second annular juncture.
The central head recess receives the above-mentioned extension of the nozzle valve. Any and all compressive or thrusting forces between the spring and the nozzle valve are transmitted via a thrusting action imposed on the nozzle valve extension in the up or down direction; all such forces are transmitted across the interface between the circular tip of the nozzle valve extension and the circular end wall of the head recess; and all such forces are transmitted between the spring and the end wall of the head recess through the body of the spring seat. The compressive or thrusting forces between the spring and the nozzle valve generate bending stresses in a bending stress zone in the body of the spring seat.
Significantly, in the just-described spring seat design, which is characteristic of EMD-type injectors, the least thick cross-section of metal in the bending stress zone, when the spring seat is viewed in cross-section taken through its central axis, is the relatively small thickness of metal extending between the above mentioned first and second annular junctures.
Such small thickness of metal is accordingly the locus of the greatest bending stresses. The portion of the spring seat head that is below or distal to the second annular juncture carries substantially no bending stresses, since that portion of the spring seat head is not tied to the nozzle valve extension, and is bypassed, so to speak, by the thrusting action of the nozzle valve extension.
Spring Seats in EMD-Type Injectors Compared with Spring-Contacting Elements of Certain Other Injector Devices.
Accordingly, the bending stress zone and bending-stress-carrying cross-section of the spring seat of an EMD-type injector extends only a small distance below the flat face or spring-receiving face of the spring seat, a distance substantially less than the wire diameter of the coil spring. This is to be contrasted with other injector devices in which the bending stress zone below the spring-receiving annular face of a stemmed, thrust-transmitting element extends more deeply below the spring-receiving face, so that a deeper cross section is available to carry bending stresses. Examples of such other injector devices are seen in U.S. Pat. No. 5,697,342 (poppet valve 86, needle valve 320); U.S. Pat. No. 5,597,118 (poppet 44); U.S. Pat. No. 5,191,867 (poppet valve 38); U.S. Pat. No. 4,758,169 (loading piston 24, central bolt 34); U.S. Pat. No. 5,056,488 (intermediate piston 5); U.S. Pat. No. 6,196,472 (spring abutment member 52,): U.S. Pat. No. 5,967,413 (spool piece 125, poppet valve 220, spool piece 325); and U.S. Pat. No. 6,029,902 (spring keeper 62).
In addition to depth of cross-section, another factor in the design of the spring seat is stress concentration at the metal surface at the inside corner formed by the intersection between the stem of the spring seat and the spring-receiving flat face of the spring seat. High surface stresses at this point can cause hairline faults, which then propagate to deeper points in the metal, leading to mechanical failure of the part. For EMD-type injectors, conventional practice has been to simply fillet this inside corner with a small radius, thereby reducing local stress concentration from what it would be at a sharply defined intersection, while at the same time avoiding any undercutting into the stem and consequent reduction of the already-small juncture-to-juncture distance referred to above. Spring seats of this design for EMD-type injectors have long operated successfully and with only occasional failures.
For many other injector devices, including those disclosed in the patents cited above, in which the bending-stress zone below the spring-receiving annular face of a thrust-transmitting element extends more deeply below the spring-receiving face, avoidance of undercutting into the stem of the element has not been perceived as necessary, and in those devices, undercutting has been employed to provide curved-wall grooves in the stem instead of using filleting.
According to the present invention, undercutting may be removed as a concern for EMD-type injectors, and undercutting rather than filleting may be employed between the stem and spring-receiving flat face of the spring seat of EMD-type injectors, provided the groove formed by the undercutting is properly shaped. Providing an EMD-type injector spring seat with an undercut groove of proper shape reduces the rate of mechanical failure as compared with a conventionally filleted EMD-type injector spring seat even though the presence of such groove slightly reduces the juncture-to-juncture distance referred to above.